1. Field of the Invention
The present invention relates to a boundary acoustic wave device that utilizes a boundary acoustic wave propagating along the interface between a piezoelectric substance and a dielectric substance.
2. Description of the Related Art
To achieve simpler and smaller package structures, boundary acoustic wave devices have recently been attracting attention instead of surface acoustic devices. In boundary acoustic wave devices, boundary acoustic waves propagate along the interface between a first and second media. The first medium is composed of a piezoelectric substance because of the use of a piezoelectric effect. For example, the second medium is composed of a dielectric substance. Their characteristics have been improved by devising the piezoelectric substance, the dielectric substance, and materials constituting IDT electrodes arranged between the piezoelectric substance and the dielectric substance.
For example, in DE 4132309A1, IDT electrodes are composed of a metal, such as Au or Ag, having a large mass, instead of Al, which is often used as an electrode material in surface acoustic wave devices. The reflection of an undesired boundary wave seems to be suppressed by regulating the thickness of each IDT electrode composed of Au or Ag to a specific range.
In a boundary acoustic wave device having a stacked structure of dielectric substance/IDT/piezoelectric substance described in WO2006/126327, the IDT electrodes have a larger density than those of the dielectric substance and the piezoelectric substance, so that boundary acoustic waves propagate with the energy of boundary acoustic wave concentrated around the IDT electrodes. Each of the IDT electrodes has a stacked structure of two metals having different densities. A metal layer having a higher density is arranged on a piezoelectric substance side. The thickness of the metal layer having a higher density satisfies 0.025λ<H<0.1λ, where H represents the thickness of the metal layer, and λ represents a wavelength determined by the period of the IDT electrodes. In WO2006/126327, the use of the electrodes results in a reduction in insertion loss.
WO2006/123585 also discloses a boundary acoustic wave device including IDT electrodes each having a stacked structure of two metals with different densities. Here, letting the density of a metal layer having a higher density be ρ1, and letting the density of a metal layer having a lower density be ρ2, ρ1/ρ2>1.8 is satisfied, and the metal layer having the density ρ1 is arranged on a dielectric substance side. Furthermore, the thickness H of the metal layer having the density ρ1 satisfies 0.034λ<H<0.5λ. This results in a reduction in the absolute value of the temperature coefficient of frequency (TCF), improving its temperature characteristics.
As described above, characteristics of boundary acoustic wave devices have been improved by devising thicknesses and types of metal materials constituting the IDT electrodes of the boundary acoustic wave devices and stacking structures of the IDT electrodes.
In each of WO2006/126327 and WO2006/123585, the metal layer having a relatively higher density and the metal layer having a relatively lower density are stacked to form each IDT electrode. Furthermore, in each of WO2006/126327 and WO2006/123585, the piezoelectric substance is composed of LiNbO3 having a negative temperature coefficient of frequency (TCF), the dielectric substance is composed of SiO2 having a positive temperature coefficient of frequency (TCF). In each of WO2006/126327 and WO2006/123585, improvement in insertion loss or a reduction in the absolute value of the temperature coefficient of frequency (TCF) is achieved by concentrating the energy of boundary acoustic wave around the IDT electrodes and biasing the energy on the metal layer side.
In WO2006/126327, although the insertion loss is reduced, the absolute value of the temperature coefficient of frequency (TCF) tends to increase. In contrast, in the boundary acoustic wave device described in WO2006/123585, although the absolute value of the temperature coefficient of frequency (TCF) is reduced, the electromechanical coefficient K2 is reduced, disadvantageously the insertion loss is increased.